The original V3 mag inherited a lot of weird design elements from the V2, such as a tapered top and a cut-out back corner.

They are actually unnecessary, so these revised magazines have a simpler shape. Also, they are sturdier and have various other little differences.

Also, they are just slightly larger, which means they fit the magwell more snugly and will not wobble or fall out. The tradeoff is that they're rather difficult to remove (especially if the retaining bar in the magwell is brand new) and are incompatible with the V2 upper.